A simple geothermal power plant includes a heat engine responsive to geothermal fluid extracted from a production well for converting some of the heat in the fluid into useful work. Construction and operation are simplified if heat depleted geothermal fluid exhausted from a heat engine were exhausted directly into the atmosphere. However, geothermal fluid almost always contains sizeable amounts of noncondensible gases such as carbon dioxide and hydrogen sulfide, and sometimes the fluid contains large quantities of other gases. In such case, exhausting a heat engine into the atmosphere is usually prohibited for environmental reasons.
Because of environmental regulations, and in order to extract more work from the geothermal fluid, the conventional approach is to exhaust the heat engine into a condenser to which cooling water is indirectly applied for the purpose of increasing the temperature drop across the heat exchanger and thus increasing the thermal head on the heat engine. In such case, the condenser must be operated at a vacuum; and to this end, a relatively large capacity vacuum pump is required because of the presence in the geothermal fluid of noncondensible gases which are pumped back into a rejection well instead of being exhausted to the atmosphere. In addition to the vacuum pump, a condensate pump is also required to pump the condensate into the rejection well. Thus, the greater thermal efficiency achieved by utilizing a water cooled condenser is achieved at the expense of more complex and costly equipment, as well as a requirement for directing a sizeable percentage of the useful work produced by the heat engine into pump power for the vacuum. In addition, the amount of gases present in the geothermal fluid extracted from a production well often increases during the life of a well with the result that the capacity of the vacuum pump required will increase with the age of the well.
It is therefore an object of the present invention to provide a new and improved geothermal power plant of the type described and to a method for using the same, wherein the overall efficiency of the plant is improved.